LOS ANGELES (Reuters) - R&B star Rihanna and rapper Nicki Minaj led nominations for the American Music Awards with four apiece on Tuesday, edging out stiff competition from pop sensations Justin Bieber and One Direction. Rihanna, 24, was nominated in the top category - artist of the year - where she will compete with Bieber, Katy Perry, pop-rockers Maroon 5 and Canadian rapper Drake. She also won nods for female soul/R&B artist and album for "Talk That Talk," and will battle Minaj, 29, in the female pop/rock artist category. ...